Large amounts of data are transmitted in ever more complex networks using optical information technology. In order to ensure the functional reliability of the networks, it is necessary to monitor the transmission reliability on a transmission path, and/or the quality of the transmitted optical signals.
Digital transmission of data is described by means of hierarchically subdivided layers in accordance with OSI reference model (Reference Module for Open System Interconnection). The lowermost, bit transmission layer (physical layer) defines the physical characteristics for information transmission. The data link layer, which is located above this, comprises protocols for checking and, if necessary, for correction of the transmitted information. This is done by using specific coding methods for error detection and error correction such as 4B/5B or 8B/10B codes, or else by using more complex methods such as FEC (forward error correction). Methods such as these also allow the bit error rate and hence the transmission reliability on a transmission path to be detected.
One disadvantage of the known methods for error identification and error correction is that critical transmission paths are identified only by the occurrence of errors. However, it is impossible to assess the quality of the link even before the errors have occurred during operation of a network. Furthermore, the known methods have an increasing proportion of redundant information as the efficiency increases, so that the effective useful data rate is reduced. Accordingly, there is a need for methods which identify the occurrence of transmission errors and/or a lack of quality in the transmitted, received signals at as early a time as possible.
There are known digital optical receivers for reception and for regeneration of optical signals after passing through a transmission path which comprise an analog input part, in which the optical signal is converted to an analog electrical signal, and a digital signal processing part, in which the analog signal is regenerated to a digital data signal with a normalized amplitude and clock information. In the analog part, the received optical signal is converted by means of a photodiode to a photocurrent, and is amplified in a preamplifier. Filtering may also be provided. The analog part of the digital optical receiver is preferably linear or is in the form of an amplitude-limiting amplifier.
The digital part of the digital optical receiver has a decision maker which is, for example, a clocked D-flip-flop, which is switched using a regenerated clock (from a clock that is recovered from the data signal). The exact timing of the original signal is thus reproduced. At its output, the decision maker produces a purely digital signal, which has a digital signal form with a standardized signal level corresponding to standards that have to be complied with. The digital signal form is in this case distinguished by minimum requirements for the rising flanks and the overshoot response of the individual pulses. A digital signal is produced at the output of the decision maker, and this digital signal can no longer be distinguished from the originally transmitted signal, except for any bit errors. A corresponding digital optical receiver is described, for example, in: E. Voges, K. Petermann (Ed.): optische Kommunikationstechnik [Optical communication technology], Section 23.7, pages 815-821, Springer-Verlag Berlin, Heidelberg 2002, whose contents are to this extent incorporated by reference in the present application.
U.S. 2002/0149821 A1 discloses a circuit which is integrated in a chip in order to control an optoelectronic transceiver, which monitors and controls a large number of functions of the transceiver. The control and monitoring functions of the transceiver are in this case mapped into specific memory areas of a memory. Flags are set when predefined limit values are overshot or undershot, and the content of these flags can be recorded via a serial interface.